Internal combustion engines are lubricated by circulating lubricating oil (or crankcase lubricant) from an oil sump generally situated below the crankshaft of the engine. To reduce the energy and fuel requirements of the engine, there is a need for crankcase lubricants that reduce the overall friction of the engine. Reducing friction losses in an engine contributes significantly to improving fuel economy.
It has long been known to use combinations of friction modifiers to obtain improved friction performance. However, conventional friction modifiers often have detrimental effects on other aspects such as lubricant stability.
A recent example of a friction reducing additive for use in automotive engine oil and/or fuel is described in International patent application No. WO 2011/107739. The friction reducing additives described in this document are the reaction product of a hydrophobic polymeric subunit selected from polyolefins, polyacrylics and polystyrenyls and a hydrophilic polymeric sub unit selected from polyethers, polyesters and polyamides. The friction reducing additives described in WO 2011/107739 are said to facilitate improved fuel economy and fuel economy retention performance in an engine oil or fuel.
In addition, oil-soluble molybdenum containing additives are also often used for their friction reducing properties. Examples of patent applications which refer to oil-soluble molybdenum additives for lubricating oil compositions include U.S. Pat. Nos. 4,164,473; 4,176,073; 4,176,074; 4,192,757; 4,248,720; 4,201,683; 4,289,635 and 4,479,883.
In particular, International patent application No. WO 00/71649 discloses use of oil-soluble molybdenum compounds at levels providing from 10-350 ppm molybdenum to the lubricating oil. When used in combination with a particular zinc dialkyldithiophosphate, a particular base stock composition and a supplementary friction modifier, it is said that enhanced fuel economy and fuel economy retention can be obtained, despite the relatively low amount of molybdenum present in the lubricating oil composition.
U.S. Pat. No. 6,423,671 ('671) relates to lubricating compositions with improved frictional characteristics which translates into improved fuel economy when the compositions are used in internal combustion engines. In particular, '671 relates to lubricant compositions containing organo-molybdenum compounds together with zinc salts, metal-containing detergents and ashless friction modifiers (referred to as surfactants). '671 states that molybdenum compounds can improve frictional characteristics but that their effect is not fully realised in the above particular compositions because of preferred absorption on moving surfaces of the non-molybdenum polar components. This competition for absorption of polar components results, for example, in a tendency for detergents to be absorbed more readily then molybdenum compounds.
'671 meets the above problem by using dispersants to form a first semi-package with the above-mentioned non-molybdenum polar components, the semi-package being made by mixing and heating the components, for example at about 90° C. for about 1-3 hours. The molybdenum component is provided in a second semi-package, and the first and second semi-packages added to an oil of lubricating viscosity.
A problem with the approach described in '671 in that it requires additional processing steps, particularly the preparation of the first semi-package. The problem of competition for absorption has also been addressed in a different way in International patent application No. WO 06/89799 by employing a detergent system of low metal ratio in a lubricating oil composition of low total base number (TBN).
Fuel economy tests are becoming more closely aligned with engine operations and so fuel economy performance is critical in all temperature regimes including the low temperatures present at engine start up.